1. Field of the Invention
This invention relates to a discharge chamber incorporated in a plasma addressed liquid crystal display device or a like device, and more particularly to a surface treatment technique for discharge electrodes of a discharge chamber.
2. Description of the Related Art
A plasma addressed liquid crystal display device in which a discharge chamber is used for addressing of a liquid crystal cell is already known and disclosed, for example, in U.S. Pat. No. 4,896,149 and Japanese Patent Laid-Open Application No. Heisei 1-217396 which corresponds to U.S. Pat. No. 5,077,553. Referring to FIG. 4, the plasma addressed liquid crystal display device shown has a layered flat panel structure which includes a liquid crystal cell 101, a discharge chamber 102 and a common intermediate substrate 103 interposed between the liquid crystal cell 101 and the discharge chamber 102. The discharge chamber 102 is formed using a glass substrate 104 and has a plurality of striped grooves 105 formed on a surface thereof. The grooves 105 extend, for example, in the direction along a row of a matrix. The grooves 105 are individually closed up by the intermediate substrate 103 to define spaces 106 which are individually separate from each other. Ionizable gas is enclosed in the thus closed up spaces 106. A rib 107 of the glass substrate 104 is disposed between each adjacent ones of the grooves 105 and serves as a barrier rib for isolating the adjacent spaces 106 from each other. A pair of parallel discharge electrodes are provided on a curved bottom surface of each of the grooves 105 and function as an anode A and a cathode K to ionize the gas in the corresponding space 106 to produce discharge plasma. Such discharge area makes a row scanning unit. Meanwhile, the liquid crystal cell 101 is constructed using a transparent substrate 108. The substrate 108 is disposed in an opposing relationship to the intermediate sheet 103 with a predetermined gap left therebetween, and a liquid crystal layer 109 is filled in the gap. A plurality of signal electrodes 110 are formed on an inner surface of the substrate 108. The signal electrodes 110 extend perpendicularly to the striped spaces 106 and make column driving units. Picture elements in a matrix are defined at intersecting positions between the column driving units and the row scanning units. In the display device having such a construction as described above, the striped spaces 106 in which plasma discharge occurs are selectively scanned in a line sequential condition while an image signal is applied to the signal electrodes 110 of the liquid crystal cell 101 in synchronism with such scanning to effect display driving of the display device. If plasma discharge occurs in a striped space 106, then the potential of the inside of the striped space 106 is put substantially uniformly to that of the anode A so that picture element selection of the row is performed. In other words, each of the striped spaces 106 functions as a sampling switch. If an image signal is applied to a picture element of a plasma sampling switch while the plasma sampling switch is in an on state, then sampling holding takes place so that lighting or extinction of the picture element can be controlled. Also after the plasma sampling switch is put into an off state, the image signal is held as it is in the picture element.
Another plasma addressed liquid crystal display device which is improved in structure of a discharge chamber so that it is easy to manufacture and is suitably used to produce a screen of a large size and/or a high resolution is disclosed, for example, in Japanese Patent Laid-Open Application No. Heisei 4-265931, which corresponds to U.S. patent application Ser. No. 07/837,971 filed on Feb. 20, 1992 and assigned to the assignee of the present patent application. The improved display device is shown in FIG. 5, Referring to FIG. 5. also the improved display device has a flat panel structure wherein a liquid crystal cell 201 and a discharge chamber 202 are layered with each other with an intermediate sheet 203 interposed therebetween. The liquid crystal cell 201 has a basically same structure as the liquid crystal cell 101 shown in FIG. 4. Meanwhile, as for the discharge chamber 202, ionizable gas is enclosed between the intermediate sheet 203 and a lower side substrate 204 to form a closed up space 205. A plurality of striped discharge electrodes 206 are formed on an inner surface of the substrate 204. Since the discharge electrodes 206 can be formed on a flat substrate by screen printing or a like technique, the productivity and the operability are high and the discharge electrodes 206 can be formed finely. A barrier rib 207 is formed on each of the discharge electrodes 206, and the barrier ribs 207 divide the plasma chamber 205 into several discharge regions which make row scanning units. Also the barrier ribs 207 can be printed by screen printing or a like technique and based, and the top ends thereof contact with the lower surface of the intermediate sheet 203. The striped discharge electrodes 206 alternately function as an anode A and a cathode K and produce plasma discharge between them.
A problem to be solved by the present invention will be described briefly below with reference to FIG. 5. In manufacture of the discharge chamber 202, a conductive paste containing, for example, Ni.sub.2 B as a principal component is used as the material of the discharge electrodes 206 while an insulating paste such as a glass paste is used as the material of the barrier ribs 207, and a process wherein the discharge electrodes 206 and the barrier ribs 207 are printed on the glass substrate and then baked is adopted. Conventionally, the barrier ribs 207 are printed and baked after the discharge electrodes 206 are printed and baked. Since a high temperature (about 600.degree. C.) baking step is interposed between the two first and second printing steps, there is a problem to be solved in that the substrate 204 undergoes thermal deformation and consequently it is difficult to achieve accurate alignment between the discharge electrodes and the barrier ribs.
In order to cope with the problem, another process has been proposed wherein, after printing of discharge electrodes, they are provisionally baked at a comparatively low temperature and then barrier ribs are printed immediately, whereafter the discharge electrodes and the barrier ribs are baked simultaneously at a high temperature. However, the simultaneous baking method has another problem in that incomplete discharge occurs very frequently with a discharge chamber manufactured by the method. It is to be noted that, while the process described above wherein high temperature baking is performed twice does not cause the problem of frequent occurrence of such incomplete discharge, it has another disadvantage in that the plasma discharge is low in uniformity and stability with a discharge chamber manufactured by the method. In order to discover the causes of such incomplete discharge, a surface analysis of discharge electrodes has been conducted using an Auger electron spectroscopic method (AES), an X-ray photoelectron spectroscopic method (XPS) and so forth. The surface analysis proved that incomplete discharge is caused by an oxide insulating substance 208 such as B.sub.2 O.sub.3 deposited on the surfaces of the discharge electrodes 206.
For example, a result of measurement of a depth profile of a discharge electrode by the AES is shown in FIG. 6. The depth profile was obtained by measuring the intensity of emitted light while continuously sputtering the surface of the discharge electrode. The sputtering time on the axis of abscissa increases in proportion to the depth of the discharge electrode layer while the emitted light intensity on the axis of ordinate increases in proportion to the concentration of each component included in the composition of the discharge electrode. As described above, the discharge electrodes are made of a conductive material which contains Ni.sub.2 B as a principal component. However, segregation occurs on the surfaces of the electrodes and the concentration of the metal component Ni which contributes to the conductivity is reduced compared with that in the insides of the electrodes while large quantities of insulating substances such as B.sub.2 O.sub.3 are present on the surfaces of the electrodes. Particularly, the discharge electrodes are substantially covered with insulating substances within the range of several hundreds nm from the surfaces thereof. Consequently, even if a predetermined voltage is applied between an anode A and a cathode K of the discharge electrodes, effective plasma discharge does not occur.